October, igi6.] 



THE ORCHID WORLD. 



253 



with a colourless fluid, the cell-sap. Cells so 

 constructed are more or less of spherical form 

 and may be seen in the young shoots of all 

 plants, in some of which they are found more 

 favourable for observation than in others, as 

 in the pith of a young growing shoot of the 

 elder, the hairs on the stamens of Trades- 

 cantia, etc. ; but in most cases a high mag- 

 nifying power is necessary for the several 

 parts above described to be clearly made out, 

 though in the footstalk of a rhubarb leaf they 

 are so large as to be readily seen by the naked 

 eye. Such it is usual to regard as the funda- 

 mental form of vegetable structure, from 

 which all subsequent developments originate. 

 In all the most highly organised plants, 

 although the cells have an independent 

 existence for a time, there are always a large 

 number of them in close contact and firmly 

 united, forming a cellular tissue, a number 

 which increases as growth progresses till in 

 the full-grown plant, even when of only 

 moderate dimensions, the number of cells in 

 the aggregate exceeds the power of ordinary 

 calculation. Nevertheless, even in the higher 

 plants isplated cells occur during temporary 

 states of existence as pollen grains, fern 

 spores, the antherozoids of mosses, etc. The 

 actual forms and sizes of the cells are as 

 varied as the structures of which they form 

 the constituent parts ; they are subject to 

 regular changes like the whole plant which 

 they build up. Only those cells are in a 

 living state which contain protoplasm ; they 

 alone can grow and give rise to new cells ; 

 cells devoid of protoplasm may, however, be 

 of service to the plant as supports and protec- 

 tion to the growing parts, or as conduits and 

 store places. 



Every new growth begins with a change in 

 the protoplasm by virtue of the vital force 

 with which it is endowed, a power that can no 

 more be accounted for or explained than the 

 states of consciousness that enables us to dis- 

 tinguish a sound from an odour, a colour from 

 a flavour, or any other ultimate fact of Nature. 

 There is a continuous movement of the 

 particles, and although extremely slow and 

 imperceptible to our limited powers of vision, 

 such a movement is inseparable from the idea 



of life. This movement results, in all the 

 higher forms of vegetation, in a division of 

 the mother cell into two others more or less 

 like itself, and these again divide in like 

 manner. As division and sub-division proceed, 

 a differentiation also takes place in the cell 

 contents ; chlorophyll granules are formed in 

 some, starch-grains, resin, crystals of various 

 kinds, etc., occur in others ; as well as a modi- 

 fication in form according as each fulfils its 

 own definite part in the economy of the plant. 



The numerous and densely crowded cells 

 form the " fundamental tissue " from which m 

 course of time, and in accordance with varying 

 requirements, different layers of tissue develop 

 differently, so that the adult plant consists of 

 diiferenfiated tissues. In general, the whole 

 mass of tissue is definitely bounded on the out- 

 side by an Epidermal layer or outer skin 

 consisting of one layer of cells. This sur- 

 rounds and encloses a rind or " cortex " of 

 several layers, whilst the centre is occupied 

 by_ a mass of cells, some of which remain 

 unchanged while others are gradually con- 

 verted into long strands. These strings of 

 tissue, the fibro-vascular bundles, usually 

 follow in their longitudinal course the direc- 

 tion of the most vigorous growth which imme- 

 diately precedes their differentiation. Not 

 only the cortical layers, but also the vascular 

 bundles and the fundamental tissues are more 

 or less differentiated, the sub-epidermal into 

 layers of a different nature ; the bundles also 

 exhibit differentiation, and generally in a still 

 higher degree. In this manner arise in the 

 higher plants Systems of Tissues. 



In two or more years' old stems of the 

 Dicotyledonous division of flowering plants, 

 also of the Gymnospermous Orders (Conifers, 

 Cycads, etc.), the component tissues are 

 arranged in concentric rings as is shown in 

 the wood of our common trees and shrubs 

 which consists chiefly of fibro-vascular bundles 

 so strongly developed by the continuous 

 formation of tissues of which they are com- 

 posed, that they finally almost replace the 

 intermediate fundamental tissue ; in the 

 leaves, the fibro-vascular bundles (veins) are 

 netted (reticulated) or otherwise more or less 

 irregularly disposed. On the other hand, in 



